My understanding on the subject is that even though you have an experimental
aircraft, you need to have a complete ARROW which includes:

Airworthyness Certificate
Aircraft Registration for Fed and State
Insurance
Pilot Operating Handbook
Weight and Balance

The foto log of construction documents your participation and supports the
51% builders requirement.  It also provides you with a foto log for future
reference when you need to make a repair in a hidden place.  Do not lose it.

The written log is also required and will privide additional information for
the FAA rep.  Again, they do not review it but scan through it to assure
them it is a log.  This document, properly prepared, indicating all your
problems, solutions, tests, recording of statistics provides you with a body
of knowledge for future reference.  ie did you do your ground fuel flow
tests to determine flow rates of each pump, how long it takes to fill up the
header tank with one or both main tank pumps on?  Did you conduct this test
with the plane at an inclined angle equal or greater than your climb out
angle?  These data serve as a baseline and can be checked with future tests
to determine pump effectiveness or deterioration thus alerting you to a
possible failure.  So you replace the pumps before the failure occurs.  Is
this information required by the FAA?  No, but it is certainly useful for
future use and peace of mind as you climb out on the first flight.

What building problems did you incur?  How were they solved?  Did you spend
too muych time on body work? or did you do it innefficiently?  What would
you do next time?

It all depends on how far you want to go in this experience; but as for me,
I welcomed the record keeping.  My plane is documented and already the logs
have been useful even only after three years and the writtin log will save
me much more time on the next aircraft project.

My understanding is that the log is there to keep a record and support the
fact that you are in fact the builder of record but it can be much more.
The choice is yours.

Good luck

Bob Smiley
N94RJ